Conventionally, X-ray radiography with an X-ray apparatus is carried out based on X-raying conditions including a tube voltage, a tube current and an emission time of an X-ray tube. In certain cases, an automatic exposure control is carried out to end (cut off) the emission of X rays from the X-ray tube when X rays emitted from the X-ray tube and transmitted through a subject have reached a predetermined amount, to keep the contrast of X-ray fluoroscopic images in an optimal state. That is, the automatic exposure control aims at keeping film density constant through an automatic control of exposure time. Specifically, the control is intended to obtain a desired film density by converting an amount of X rays transmitted through a subject into electric signals, and cutting off X rays when this amount of electricity reaches a fixed value (see Patent Document 1).
In certain cases, a flat panel X-ray detector (hereinafter abbreviated as “FPD”) is provided in place of the film in the X-ray apparatus noted above. The FPD has a sensitive film laminated on a substrate, detects X rays incident on the sensitive film, converts the detected X rays into electric charges, and stores the electric charges in capacitors arranged in a two-dimensional array. The stored electric charges are read by turning on switching elements, and are fed as X-ray detection signals into an image processor disposed at a subsequent stage. Then, the image processor provides X-ray fluoroscopic images based on the X-ray detection signals.
Further, an X-ray apparatus with this FPD may have a sequential shooting function as well as the above-noted automatic exposure control function. That is, the above-noted automatic exposure control is carried out at each shooting time of the sequential shooting. Specifically, when radiography (X-ray emission) is started and an amount of transmitted X rays reaches a predetermined value, the X-ray emission is stopped by the above-noted automatic exposure control to complete the current radiography. The automatic exposure control is carried out for a next radiography also. Thus, a plurality of consecutive X-ray fluoroscopic images are acquired under automatic exposure control operating constantly (at each emission).
Before starting a sequential shooting radiography, the operator (e.g. an X-ray radiographer) needs to set a radiographic collection mode to this X-ray apparatus, such as a subtraction mode, a mode of moving a radiographic system or a fluoroscopic table, or, conversely, a non-moving mode, in order to determine whether to carry out the sequential shooting radiography in a state of locking (fixing) X-raying conditions or in a state of automatic exposure control. The subtraction mode is, for example, for acquiring subtraction images (difference images) based on differences between mask images (images before injection of a contrast medium) and live images (images after injection of the contrast medium). The mode of moving a radiographic system or a fluoroscopic table is, for example, for radiographing a plurality of sites to be imaged of a subject successively by moving the radiographic system or the fluoroscopic table. The non-moving mode is, for example, for radiographing a single site to be imaged of a subject without moving the radiographic system or the fluoroscopic table.
[Patent Document 1]
Unexamined Patent Publication No. 2000-173795 (page 2, FIG. 11)